1. Field of the Invention
The present invention relates to a fuse used for protecting the electric circuit of an automobile and others, and more particularly relates to a delayed-fusion fuse, the durability of which is enhanced with respect to a transient current applied to the fuse.
2. Description of the Related Art
In general, the pre-arcing time-current characteristics of a fuse used for protecting the electric circuit of an automobile and others are distinguished between the high current region H and the low current region L, as shown in FIG. 7. The high current region H of the former is a region in which fusion is caused by a burst current in the case of a dead short-circuit, and the fusion advances in a relatively short period of time in which several seconds passes from the heat generation to the fusion. Therefore, the electric circuit is shut off before the covered portion of an electric wire is scorched or the casing is melted.
On the other hand, the low current region L of the latter is a region in which a relatively long period of time is required from the generation of heat to the fusion of a fuse. In this region, when a rare short-circuit is caused, the fusible section is overheated for a long period of time, so that the covered portion of an electric wire scorches and smokes, and further there is a possibility that the casing is melted. For example, a transient current several times as high as the steady-state load current flows in the load circuit of an electric motor when the motor is started. In the case of a power-window motor, a motor lock current several times as high as the steady-state load current flows in the load circuit when the windowpane is fully closed or opened. That is, a current exceeding the steady-state current frequently flows in the low current region. Therefore, in the load circuit of an electric motor, a delayed-fusion fuse is used, which is not fused with respect to a transient current exceeding the steady-state current and also with respect to a motor lock current.
Conventionally, this type delayed-fusion fuse is disclosed, for example, in Japanese Unexamined Utility Model Publication No. 59-66844 and others, by which the pre-arcing time-current characteristics are improved in the following manner. A low fusing point metallic chip is held in an intermediate portion of the fuse body made of fusible metal of high fusing point, and by the diffusion of the low fusing point metallic chip, an alloy is generated. By the generation of the alloy, the pre-arcing time-current characteristics are improved.
As shown in FIG. 8, the delayed-fusion fuse 100 is integrally composed of an electrically conductive metallic plate, and a pair of electrically connecting terminal sections 103 are provided on both sides of a narrow fusible section 101. A fusing section 102 holding a low fusing point metallic chip 110 providing a heat absorbing effect is formed at an intermediate position of this fusible section 101. The delayed-fusion fuse 100 is folded into a reverse U-shape around this fusing section 102.
Because of the action of the low fusing point metallic chip 110, a time lag from the input of a current to the fusion of the fuse can be maintained in the following manner. For example, suppose a low current flowing during the starting of a motor is higher than an allowable continuous current of an electric wire used in the electric circuit. However, further suppose the current is in a range of fusion of the fusible section 101. Even when the fusible section 101 is heated and generated heat concentrates on the fusing section 102, the heat is transmitted to and absorbed by the low fusing point metallic chip 110, the heat conductivity of which is high, and the heat absorption effect of which is excellent. In this way, the time lag can be ensured. In other words, an allowable range of the fusing section 102 which is not instantaneously fused even when a transient current flows in the fuse, is extended by the low fusing point metallic chip 110, so that the property of delayed fusion can be ensured.
In this case, when the property of delayed fusion is excessively provided, the electric wire is put in an overheating condition for a long period of time, and the covered portion of the electric wire scorches and smokes as described before. Therefore, at a point of time exceeding the predetermined time lag, the delayed-fusion fuse 100 must be fused away. With reference to FIG. 9, this principle will be explained below. In accordance with the transmission and absorption of heat generated when a rush current flows, the temperature of the metallic chip 110 is raised (FIG. 9a). When the temperature reaches the low fusing point, the low fusing point metallic chip 110 is fused so that the fused metal diffuses into the fusible section 101, and a solid solution, the fusing point of which is lower than that of the original fusible section 101; that is, an alloy layer 111 is formed (FIG. 9b). Because of this, the fusible section 101 is fused away after the predetermined time lag has passed (FIG. 9c).
However, in the conventional delayed-fusion fuse described above, the low fusing point metallic chip directly comes into contact with the heated fusing section. Therefore, when a transient current flows which is in the low current region and higher than the allowable continuous current (that is, when a current flows which exceeds the steady-state current, for example, when a motor-lock current flows), generated heat in the fusing section is transmitted to the low fusing point metallic chip although the transmitting time is short. When this transient current flows periodically, the low fusing point metallic chip gradually diffuses, and the pre-arcing time-current characteristics are deteriorated, so that the predetermined durability cannot be reliably ensured.